Metering and Instrument Circuits

Continuity Tester

A continuity tester is very useful for checking printed-circuit boards. It indicates a sound connection by a squeak from a buzzer there is, therefore, no need to continually watch a meter.

The voltage across test terminals TP1 and TP2 is only 80 mV. That is not sufficient to test diodes, but it obviates the risk of damage to electronic components.

The design consists of a comparator IC-1, and an astable consisting of T1 and T2. whose frequency is around 1250 Hz for a supply voltage of 3 V. The astable is actuated the moment the output of IC-1 (pin 6) goes logic high. This happens when the resistance between TP1 and TP2 is low, so that the voltage at pin 2 of the comparator is lower than that at pin 3, which depends on the setting of P1. The circuit is powered by a 3-V battery (two 1.5 V cells in series), but may be maximum 12 V. At such a high supply voltage, it may be that the tone of the buzzer is too high: in that case, the values of C1 and C2 should be increased.

If a Type TLC271C is used instead of a TLC251 for IC-1, the supply voltage must be not lower than 3 V. At 3 V, the circuit draws a current of about 1.4 mA.

Continuity Tester

Continuity testing overview

  • Continuity is the presence of a complete path for current flow. A circuit is complete when its switch is closed.
  • A digital multimeter’s Continuity Test mode can be used to test switches, fuses, electrical connections, conductors and other components. A good fuse, for example, should have continuity.
  • A DMM emits an audible response (a beep) when it detects a complete path.
  • The beep, an audible indicator, permits technicians to focus on testing procedures without looking at the multimeter display.
  • When testing for continuity, a multimeter beeps based on the resistance of the component being tested. That resistance is determined by the range setting of the multimeter. Examples:
    • If the range is set to 400.0 Ω, a multimeter typically beeps if the component has a resistance of 40 Ω or less.
    • If the range is set 4.000 kΩ, a multimeter typically beeps if the component has a resistance of 200 Ω or less.
  • The lowest range setting should be used when testing circuit components that should have low-resistance value such as electrical connections or switch contacts.

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